Many releasable door hold-open devices are known in the art for automatically releasing a held-open door when a predetermined condition is detected, such as the existence of smoke or heat. These devices are often used with fire doors, which must be closed in the event of a fire to inhibit spreading of the fire.
Most prior devices employ a solenoid that must be continually energized to hold open the door. Representative examples of such devices are disclosed in representative U.S. Pat. Nos. 3,729,771, 3,771,823, 3,905,063 and 4,040,143. When smoke or heat is detected, or if current to the device is interrupted, the solenoid is de-energized, thus automatically releasing the door and allowing it to close under the force of a door closer. This implementation has the distinct disadvantage of requiring the electric actuator to be continuously energized to hold the door in the open position. A continuously energized actuator draws a large continuous electrical current, which wastes electricity, decreases the life of the actuator and makes battery operation impractical. Other shortcomings of door hold-open devices employing remote power sources include the expense and complication of providing wiring to the devices and the necessity of resetting multiple devices after an alarm condition.
Door hold-open devices that employ a local power source, such as a battery, can solve many of the foregoing problems. However, battery-powered door hold-open devices suffer from the shortcoming of having a short battery life. This is due to the constant current which must be used to monitor for an unlatch condition, as well as the larger current which must be used to actuate the motor or solenoid in the latching mechanism. For example, the door hold-open device disclosed in U.S. Pat. No. 5,072,973, which is incorporated herein by reference, can be actuated about 500 times before the battery becomes inoperative. Accordingly, it is desirable to provide a door holder with a means for minimizing power consumption during inactive periods.